An endless drive means of this kind is used in internal combustion engines, such as spark-ignition engines and diesel engines in motor vehicles, e.g. in passenger cars, trucks or similar land vehicles. However, they are also used in water- and aircraft.
Here, the endless drive means are arranged so as to transmit power between different shafts, e.g. between a crankshaft, one or more intermediate shafts and/or one or more camshafts.
Numerous tensioning devices are known from the prior art. Thus, Germany Utility Models DE 20 2006 018 836 U1 and DE 20 2007 004 988 U1 and German Laid-Open Application DE 10 2007 036 119 A1 disclose hydraulically operated tensioning devices which are used in internal combustion engines. Such tensioning devices have a housing which can be fixed on an engine component of the internal combustion engine. Conventional solutions are implemented by enabling the housing of the tensioning device to be screwed onto or into an engine component, such as the engine block. A cylinder is inserted into the housing, in particular into a bore in the housing and arranged in the housing in such a way that it can be extended from the said housing. To extend the cylinder out of the housing, a pressure chamber present between the cylinder and the housing can be filled with a pressure-transmitting fluid, that is to say a pressure fluid. A supply opening is provided in the housing for this purpose.
If a pressure fluid, such as oil, is now fed into the interior of the pressure chamber, the cylinder is forced to extend out of the housing of the tensioning device. As it does so, it exerts pressure on a tensioning rail that deflects the endless drive means.
In addition to the force exerted on the cylinder by the pressure fluid, a spring present in the pressure chamber can also apply force to the cylinder.
The pressure fluid is also intended to be able to exert a damping effect on the tensioning rail, and there is therefore a nonreturn valve in a supply opening in the pressure chamber. The pressure fluid, such as oil, e.g. a hydraulic oil, can then damp shocks transmitted by the endless drive means, e.g. those during cold starting processes.
To prevent the pressures prevailing in the pressure chamber from becoming too high, the previous practice was to choose the gaps between the cylinder and the housing in such a way that a certain leakage occurred and was even desired.
In the course of the further development of existing tensioning devices, there was, however, a need to be able to reduce the design delivery rate of pressure fluid pumps feeding pressure fluid to the pressure chamber. The leakage flows were therefore lower or smaller. As a result, however, another way had to be found to prevent excess pressure within the housing, i.e. between the housing and the cylinder, namely the pressure chamber. For this reason, additional pressure relief valves were increasingly installed in tensioning devices.
Tensioning devices with nonreturn valves and pressure relief valves are known from the prior art. Attention is drawn here to publications DE 10 2004 040 222 A1, DE 69908702 T2, DE 20202665 U1 and DE 10 2004 043 727 A1. While, in the two first-mentioned publications, a pressure relief valve, on the one hand, and a nonreturn valve, on the other, were provided at the opposite ends of the cylinder, DE 20202665 U1 takes the approach of providing both types of valve on one side of the cylinder. DE 10 2004 043 727 A1 also separates the two types of valve strictly from each other in terms of installation space.
However, these already known solutions from the prior art have the disadvantage that the installation space within the cylinder is greatly reduced and hence that only relatively short springs can be used between the cylinder and the housing, entailing disadvantages in terms of the spring characteristic. Moreover, the pressure chamber as such is reduced in size. Such solutions known from the prior art can thus only be implemented when the installation space is sufficiently large.
It has been found that installing a pressure relief valve and a nonreturn valve separately from each other requires more installation space than is normally available. Moreover, existing pressure relief valves have the disadvantage that there is an increase in the oil consumption of the tensioning device.
Alternative solutions have been made public by DE 19957527 A1 and DE 9409155 U1. In these two publications, combination valves are used as a nonreturn valve and pressure relief valve on one side of the cylinder. DE 9409155 U1 is considered to be the prior art that defines the generic type.
However, these known tensioning devices have the disadvantage that they still require a large amount of installation space and, furthermore, the pressure relief valve has a large mass, which prevents the use of these two known solutions on internal combustion engines of lightweight construction. Moreover, the accuracy of response of the known pressure relief valves in these documents is not sufficiently high.
It is therefore the object of the present invention to avoid the disadvantages of the prior art and to provide a tensioning device which has a pressure chamber that can be filled with pressure fluid, wherein the pressure chamber can be used as a damping element and, at the same time, a safeguard against excess pressure and a means of preventing backflow of oil is provided at the same time as high accuracy of response and low weight.